1. Field of the Invention
The present invention relates to an image forming apparatus, such as a laser beam printer, a photocopier, a plain paper facsimile machine, etc., and a toner image fixing device.
2. Discussion of the Background
As an example of fixing devices for fixing a toner image on an image transfer sheet such as a sheet of paper as a permanent fixed image, a heat roller fixing device is widely used.
FIG. 1 is a schematic view illustrating a heat roller fixing device as an example of background art. The heat roller fixing device of FIG. 1 is typically used for fixing a relatively thick toner image, such as a fixed toner image required to be glossy such as a full color toner image. Referring to FIG. 1, the heat roller fixing device provides a fixing roller module 5, a backup roller module 6, a cleaning roller 13, and an oil coating roller 14. The fixing roller module 5 includes a halogen heater 1 as a heat source, a fixing roller having a hollow metal core 2, an elastic layer 3 on the hollow metal core 3, and a release agent layer 4 as the outermost layer. Similarly, the backup roller module 6 includes a halogen heater 7 as a heat source, a backup roller having a hollow metal core 8, an elastic layer 9 on the hollow metal core 8, and a release agent layer 10 as the outermost layer. The halogen heater 7 in the backup roller is optional and may be omitted.
An image transfer sheet 12 bearing a toner image 11 is transported into a nip formed between the fixing roller module 5 and the backup roller module 6, and the toner image 11 is softened or melted and fixed onto the image transfer sheet 12 by heat and pressure by the fixing roller and the backup roller. The cleaning roller 13 contacts the fixing roller module 5 to remove toner adhered on the surface of the fixing roller module 5. The oil coating roller 14 also contacts the fixing roller module 5 to apply oil as a release agent to the surface of the fixing roller module 5 to prevent the image transfer sheet 12 from twining around the fixing roller module 5. The oil also prevents the melted or softened toner from adhering to the fixing roller module 5.
Overlaying four color toners at most, such as of cyan, magenta, yellow, and black toners, generally forms a full color image. To form a quality image, color toners are sufficiently melted and fixed so that each color toner can transmit a specific spectrum of light and thereby a merged spectrum generates a preferable color. For that reason, the hollow metal core 2 of the fixing roller module 5 is sometimes structured to have a large amount of heat capacity, and the elastic layer 3 is structured to sandwich the toner image 11 together with the backup roller module 6 for supplying sufficient heat.
The heat roller fixing device of FIG. 1 generally uses a relatively large amount of release agent because of a relative high temperature of the heat roller. However, when such a relatively large amount of the release agent is applied to the fixing roller module 5, the release agent generates a sludge-like mixture together with toner powders adhered on the fixing roller module 5, fiber and/or dust of paper, calcium carbonate, etc. Such a sludge-like mixture sometimes soils the fixing device, and may also soil the hands of a user or a service person of the image forming apparatus in which the fixing device of FIG. 1 is installed during a replacement operation of expendable equipment, such as the oil coating roller 14. The sludge-like mixture sometimes also hampers workability of the replacing operation of the expendable equipment as well. Therefore, a demand for a fixing device that uses a relatively small quantity of release agent in comparison with the background art is increasing.
As another example, Japanese Laid Open Patent No. 9-160410 describes an image forming apparatus installed with a fixing device having a heat roller pair constructed by a fixing roller and a backup roller. Each roller of the heat roller pair has a gear as a single piece on an axis. The fixing device also provides a belt, spanning parts for spanning and driving the belt, a belt charging device, a belt discharging device, and a preliminary heating device upstream from the heat roller pair. The fixing device further provides a motor for driving each of the heat roller pair via the gears and the preliminary heating device.
In this example, additional elements, such as the belt, the spanning parts, the preliminary heat source, the belt charging device, etc., are required, and therefore the production costs may increase. Further, each roller of the heat roller pair is driven via the gears fixed to the rollers by the single motor, so that an error in hardness of the elastic layer and/or an error of a diameter thereof may cause a difference between the surface velocities of the rollers. Consequently, such a difference between surface velocities may cause an image transfer sheet to become crinkled.
The present invention has been made in view of the above-discussed and other problems and has as one objective to overcome the above-discussed and other problems associated with the background methods and apparatus. Accordingly, one object of the present invention is to provide a novel image forming apparatus and a novel fixing device that can fix toner images on a image transfer sheet without release agent or with a relatively small quantity of release agent.
Another object of the present invention is to provide a novel image forming apparatus and a novel fixing device that can decrease chances of an image transfer sheet becoming crinkled.
A further object of the present invention is to provide a novel image forming apparatus and a novel fixing device that can reach a ready-to-fix state in a relatively short time.
To achieve these and other objects, the present invention provides a novel image forming apparatus and a novel fixing device including a fixing roller and a backup roller configured to press the image transfer sheet against the fixing roller, wherein either one of the circumferential hardness of the fixing roller and the circumferential hardness of the backup roller is configured to be harder than the other. The image forming apparatus and the fixing device further include a first gear coaxially mounted on the roller having the harder circumferential surface, a one-way clutch coaxially mounted on the other roller, and a second gear coaxially mounted on the one-way clutch. The first gear and the second gear are configured to be engaged when the backup roller presses the image transfer sheet against the fixing roller, and one of the first gear and the second gear is driven by a power source.